Add processing to update native methods CRC32 (#67)

***NO_CI***

Author: josesimoes

source/MetadataProcessor.Core/nanoAssemblyDefinition.cs

@@ -5,6 +5,7 @@
 //
 
 using System.IO;
+using System.Linq;
 
 namespace nanoFramework.Tools.MetadataProcessor
 {
@@ -89,8 +90,20 @@ public void Write(
             // keeping this here for now, just for compatibility
             writer.WriteUInt32(0);
 
+            var nativeMethdodsCount = _context.MethodDefinitionTable.Items.Count(method => method.RVA == 0 && !method.IsAbstract);
+
             // native methods CRC32
-            writer.WriteUInt32(_context.NativeMethodsCrc.Current);
+            // this is used by other tools to check if the assembly has native implementation 
+            // (like the deployment provider in the VS extension)
+            if (nativeMethdodsCount > 0)
+            {
+                writer.WriteUInt32(_context.NativeMethodsCrc.Current);
+            }
+            else
+            {
+                // this assembly doesn't have any native methods implemented
+                writer.WriteUInt32(0);
+            }
 
             // Native methods offset
             writer.WriteUInt32(0xFFFFFFFF);